This diagram explain why parries should be short and compact motions.

I noticed that people try to use large or fancy motions as parries when they begin studying.
Let this diagram explain to you why parries, or any action that's in reaction to someone else's move, need to be small and compact. This also explains why attacking is so much more valuable than reacting. The German masters recommend that you be in vor instead of nach, and this is why.
This explains that you have to recognize what's happening, decide what to do, and then act in the same amount of time that the attacker simply has to act. Big motions take too long and fancy ones require too much precision to be reliable.